We have a domain registered under the name vatonline.ey.hu and we have been told that the SSL certificate issuer is you.
Could you please inform me on how to get of a copy of this certificate? What information would you need from us in order for you to send a copy?
This information is visible to anyone who visits your web site (including with a regular browser—check the security information and/or web developer tools), and you can access it yourself this way without needing to ask the certificate authority for a copy.
As @JuergenAuer indicates, the certificate information is provided from your own server, which means that there must also be a copy of it somewhere on your server itself. It looks like your server is running Microsoft-IIS/10.0. I have no experience with this software, but from documentation that I see online, it looks like you should be able to access your certificate configuration by running the IIS Manager application on your web server.